In Wideband Code Division Multiple Access (WCDMA), the traditional way to demodulate a signal is to apply a Rake type of receiver. A Rake receiver combines energy received at different delays and from different antennas. This is done by despreading the data at the delays where significant amounts of energy can be found. The combining weights are typically directly determined from the channel estimates so that each despread path, a so-called Rake finger, is given a weight equal to the conjugated channel response at that delay. Rake receivers work well when the spreading factors are large, so that there is little correlation between the different Rake fingers.
With the introduction of Enhanced uplink (EUL) spreading factors (SF) as low as 2 are used, meaning that significant correlation may exist between different Rake fingers. In these scenarios the achievable rate is limited by self interference and multi user interference. For these situations, so-called Generalized Rake (GRake) receivers were designed. The GRake method estimates the correlations between Rake fingers and uses this correlation to modify the weights. Furthermore, with GRake there is also an option to define so-called interference suppression fingers that does not contain signal energy but can be used to suppress interference. The GRake correlations, or equivalently, the impairment matrix, can be estimated in several ways. The so-called parametric GRake utilizes knowledge of the used scrambling codes and channelization codes to derive code averaged weights. As input to the model, despread data from the pilot channel, the Dedicated Physical Control CHannel (DPCCH), is used. Another flavour of GRake, also often referred to as non-parametric GRake, uses an unused channelization code to estimate the impairment.
The parametric GRake method is known to perform well in suppressing self-interference from one user transmitting at very high rate with one user scenarios. This is especially true in DownLink (DL). The parametric GRake was first developed for DL and later modified for the UpLink (UL), where the performance also has been verified to be acceptable. The non parametric GRake has also shown very promising performance in UL simulations and also in tests in real hardware. However, in certain UL situations, the gain performance has been found to be somewhat less than expected. This has particularly been found in some situations where strong signal channels are separated by approximately one chip.